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11NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

• Technical Memorandum 33-730

Ionizing Mechanisms in a Cesium PlasmaIrradiated with a Ruby Laser

K. Stimada

L. 6. Robinson

(NASA-CR-14313 1 ) IONIZING MECHANISMS IN A N75-27899CESIUM PLASMA IRPACIATFD WITH A RUBY LASER(Jet Propulsion Lab.) 15 p HC $3.25

CSCL 201 ITnclas63/75 29265

JET PROPULSION LABORATORYCALIFORNIA INSTITUTE OF TECHNOLOGY

PASADENA, CALIFORNIA

June 1, 1975

•t^

^I

NATIONAL AERONAUT ICS AND SPACE ADMINISTRATION

Technical Memorandum 33-730

Ionizing Mechanisms in a Cesium Plasma

Irradiated with a Ruby Laser

K. Shimada

L. B Robinson

fL __.

JET PROPULSION LABORATORY

CALIFORNIA INSTITUTE OF TECHNOLOGY

PASADENA, CALIFORNIA

June 1, 1975

L.&-

J

JPL Technical Memc

PREFACE

The work .inscribed in thin report was performed by the Applied MechanicsDivision of the Jet Propulsion Laboratory.

Dr. L. 13, Robinson is a consultant for the Jet Propulsion Laboratory,His permanent address is the University of California, School of Engineeringand Applied Science, Los Angeles, California 90024.

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echnical Memorandum 33-71" v

CONTENTS

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

11. Theoretical Considera ,ns . . . . . . . . . . . . . . . . . . . . . . . . . 2

Ill. Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

IV. Analysis of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

\' Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

R, arences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

TABLES

1. Energies involved in absorptions in cesium . . . . . . . . . . 9

2. Conversion factors . . . . . . . . . . . . . . . . . . . . . . . . . . 9

FIGURES

1. Schematic diagram of the LPD corverter . . . . . . . . . . . . 10

2. Measuring circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3. Oscillographs showing the LPD output 12

4. Collected charge vs collector voltage at low colle^tor

temperatures with collector heat current I CH = 2.6A . . . . 13

5. Collected cha r ge vs collector voltage with I C ; I = 7. 5A(collector temperature = 950°1f) . . . . . . . . . . . . . . . . . 14

ABSTRACT

A cesium filled diode--laser plasmadynamic (LPD) converter was

built to invrsttgate the feasibility of converting; laser energy to e' ;ctrical

energy at large power leveis. Experiments were performed with a

pulsed ruby laser to determine the quantity of electrons anJ cesium

ions generated per pulse of laser beam and to determine the output

voltage. A current density as high as 200 amp/cm 2 frurn a spot of

approximately 1 mm area and an open circuit vo l '.age as high as 1. 5

volts have been recorded. A qualitative theory has been developed in

order to expla n these results,

In the operation of the device, the laser beam evaporates some of

the _esium and ionizes the cesium gas. A dense cesium plasma .s

subsequently formed to absorb further the laser energy.

Results suggest that the simtiltanoous absorption of two ruby laser

photons by the cesium atoms plays an important role in the initial ioniza-

tion of cesium. Inverse bremsstrahlung absorption appears to be the

dominant mechanism in subsequent processes. Recombinations of

electrons and cesium ions appear to compete favorably with the simul-

taneous absorpt i on of two photons. A better understanding of charge

particle transport mechanism is required to improve further the LPD

energy conversion.

V i JPL Technical Memorandum 33-730

IONIZING MECHANISMS IN A CESIUM PLASMAIRRADIATED WITH A RUBY LASER

I. INTRO._'UCTION

A cesium-filled diode; laser plasmadynamic (LPD) converter was built to

investigate the feasibility of converting laser energy to electrical energy at

large power levels ( >1 kW/cm 2 ). Any energy conversion device requiressimultaneous generation of output voltage and output current which, in theLPD converter, is carried by cesium ions and electrons.

A schematic diagram of the LPD converter is shown in Fig. I. The diode isforme,:? with a cup-shaped electrode (emitter) containing cesium liquid and asemi-spherical electrode (collector) above the emitter. The diode is enclosedin a stainless cylinder which is provided with: (1) sapphire windows, one forthe laser beam and the other for an optical observation, (2) an eight-pin lei i-

through foi connection to collector electrode, collector heater and d therrr. )-

couple. The collector material is stainless and a sheathed heater is brazed on

it. The collector, which has a dome radit , s of 0.794 cm, has a hole with a

diameter of 0, 476 cm through whi... , the • ncident laser beam is introduced,

striking the center of the cesium reservoir at a 45-deg angle. The cesium

reservoir is made of copper which is externally connected to a temperature-

controlled copper block. To avoid undesirable condo isation of cesium at placesexcept !:,e reservoir, the reservoir is kept at the .`iest temperature (^--50°C!in the system.

Experiments were performed with a pulsed ruby laser to determine tooth thequantity of cesium ions and electrons generated per pulse of laser beans andthe output voltage. The principle of the converter operation is to: (1) generatehigh-density cesium vapor at or near the focal spot of the laser, (2) photoionizethe gas, and (3) separate ions and electrons by two electrodes that are at twodifferent work funL,.ons.

PL Technical Memorandum 33-730 1

1

Since it is appropriate to use coulomb-volt curves for pulsed operations, suchcurves have been obtained under various laser inputs and collector tempera-tures. The circuit used for these measurements is shown in Fig. L, andtypical oscillographs are s';uwn in Fig. 3 in which the input laser pulse (uppertrace) and the output current (lower trace) are ind;cated. Note that the change

iof current polarities occurred as the polarity of voltage :%-as reversed. Alsothe current tended to saturate at an applied voltage on the order of a volt, con-firming the low level of energy possessed by generated charge carriers. Theelectror current (in negative quadrant) showed faster response time comparedwith that of the ion current. A close examination showed practically a one-to-onecorrespondence between the laser ; pulse and the .lectron current spikes. How-ever, this t,pc of relationship xas not observed in the ion current. Note alsothat the curve moved to the right, i. e. , there Nvas a positive current when thedevice \vas shorted, which occurred v, ,hen the collector \vas heated to 950'x.This was expected since the collector work function should be higher than thatof the emitter (cesium reservoir) and the collector tended to reject electronsand collect ions. However, the quantity of current or the voltage was stillmuch less than what was expected. Possible causes are the use of a ruby laserwhose wavelength is not smail enough for a one photon ionization process, thepulsed operation, or the large interelectrode gap of this LPD converter. Apartial analysis of the cesium ionization is described below.

II. THEORETICAL CONSIDERATIONS

It is evident that the following events occur when the laser radiation is incidentupon the pool of cesium. The laser beam Evaporates some of the cesium withionization to form a dense plasma; this is followed by absorption of the laserenergy in the plasma. The ionization can be one, two, or three photon pro-cesses. Eventually ionization will occur. A part of the generated electronsare accelerated by the inverse bremsstrahlung mechanism so that they excitecesium neutrals and subsequently ionize excited species. Recombinationsbetween cesium ions ant. electrons also occur in the plasma. Coincident withall of these competing processes, net current is produced by the passage of

Z JPI. Technical Memorandum 31-7 W

the ions and electrons in the appropriate electrode trader the influence of thefield in the interelectr- ► de region. Some consideration will now Le given to therelative importance of the above processes, based on the available data andtheory.

Table 1 provides a guide as to processes which are energetically possible.• Some of the important transitions from the ground state to excited states are

shown. We see that the work function of cesium is verl close in magnitudeto the energy of the laser photon. The simultaneous absorption of two photonscorresponds to a transition from the ground state to the 91.) level. Only afraction of an eV separates this state and the ionized state.

It has been determined experimentally that the excitation of the 65 1/2 — 9D3/2transition in cesium has been induced by intense light from a ruby laser l .The 91)5/2' 3/2 levels have 'erm values which are twice the frequency of theruby photons.

Ionization of cesium atoms can occur in several possible ways. Likely onesare: (a) simultaneous absorption of three photons, (b) simultaneous absorptionof two photons, followed by ionizing collisions with energetic electrons, (c) exci-tation of neutrals by energetic electrons followed by simultaneous absorption oftwo photons, (d) sequential excitation of neutrals by energetic electrons followedby absorption of one photon, (e) sequential excitation of neutrals by energeticelectrons followed by ionization by electrons. Results obtained so far can ruleout some of the above processes as being unlikely, biA no single process hasbeen identified to be a dominant one.

1II. EXPERIMENTAL RESULTS

Some of the results obtained so far are shown in Figs. 4 and 5. Figure 4 showsthe results of two different experiments, performed in essentially the samemanner. Different values of the input laser energy were used as shown o:, thegraph. The results shown in Fig. 5 were obtained when the collector electrode

JPL 'Technical Memorandum 33-730 3

was heated to appruxLmately J50° 1{. The results show that (1 ) a few hundredmicrucoulombs represent the typical saturation current collected (. ig. 4),and (2) t.s much as uue to two thousand microcoulumbs were collected with tr.a-qpecial operating conditi as (Fig. 5!.

Table 2 lists some conversion factors which are important in the followingdWcussion.

IV. ANALYSIS OF RESULTS

B. A. Tozer 2 developed the following expression to account for the production

of No electrons in laser pulse tine T by the simultaneous absorption of N.photons in volume V irradiated by the laser beam:

N Ll(n Vat) II

No _ VNT exp(- n LQt) — I (1)

Where N is the number of atoms cni i in volume V, t is the reciprocal of thephoton freq uency t, tit/ is the photon flux (no cm 2 sec 1 ) and a is the crosssection for the excitation of the atom b; the simultaneous absorption of NV

photons.

First we shall consider the possibility of a three photon process, i. e. , N L = 3.If we select the number 800 µC to represent the total number of electronstypically -ollected in the experiments, and with v = 10 -16 cml , a value oftenquoted, 3 one sees that for a pulse of 100 µsec duration, the product VN mustbe about 10 3? in order to obtain N o = ; X 10 1 5 electrons, corresponding to80u µC. If one considers that recombinations have some effect, the product VNmust be larger. A produce of VN - 10 32 is equivalent to a plasma atom densityof 1C29 cni -3 throughout a liter volume. This is highly un.ikely. Therefore,one can rule out any significant contribution from a three photon process in thepresent experiments.

4 JP1. Technical Memorandum 33-730

P W-

If we _onrider the two-photon process, we note that it. Eq. (1) the requiredVN produced is reduced by a factor of about 10 9 from the previous case.This present result corresponds to a plasma atom density of 1020cm-3throughout a liter volum Under any circumstances, the required VNproduct )s less by a factor of 10 9 for the two photon process than for thethree photon process. 1 he conclusion is that one would e:.pect the simultan-eous absorption of two ruby laser photons to play a significant role in thrionization mechanism.

With the two photon absorption process, one could have pre-excitation of thecesium atones or post-ionization. In order to determine the relativeimportance of the abo r e two processes, one needs the excitation (by ele^trons)cross section from the ground state, the ionization cross section from excitedstates, and the lifetimes of excited stars.

Whenever plasma electrons are exposed to laser radiation, it is likely thatenergetic electrons will exist ir. profusion. The absorption of photons by theinverse bremsstrahlung me .nan'-lm is a highly probably proc. ss. R. D. Kerr

et al. 4 state "It is likely that when ionization is appreciabie, conventional

inverse bremsstrahlung absorption will take over as the main heating (andhence ionizing) mechanism. " Preliminary calculations in the present caseindicate that the above assertion is borne out.

In the p- esent ease an appropriate expression for the inverse bremsstrahlungabsorption coefficient, 5 K V is

2nKV = 3. 69 x 10 8 V3 Z e 112 ' (2)

a

where n is the electron density, T the electron temperature, and v thelaser photon frequency. K,, has the dimensions ofcm 2 /cm 3 or cm' 1 and is

JPL 'Technical Memorandum 33-730 5

°1

like a mean free path. We can calcula.e i v for assumed electrun

itemperatures and densities, and hencr obtain ar, equi% , alent cross section

oe for the process according to the followings eq ,ivalence relationship.

K t, = n Iv (3)e

In Eq. (2), if we take

n = 10 16 cm -3 and T = 2000•Ke e

as being typical, .. • e obtain

K V = 1. 012 X 10 - 5 cm- 1

and cue , the effective cross section for Inverse bremsstrahlunE, absorption, is

given by,

or = 10 5

X 10-16 CM e

With such an enormous cross section, this mechanism is likely to take over.

Finally, something needs to be said about recombinations. C. J. Chen et al. f'

measured , ecombina'ion coefficients as a function of electron temperatureand density :or cesium plasma. The recombination coefficient, a,in cm sec- 1 is defined in the following equation:

dn e = _an

Z (4)_

dt e

It

6 JPL Technical Meriorandum 33-730

a anci tht- recombination cross section, o r , are related as follows

a a c r v, (5)

where v means electron speed and the bar means average. If we take the

average speed to be about 2 x 107 cm/sec, a rough calculation give, o r to

be about 5 x 10 - 16 cm 2 , a significant cross section. They ( meadured values

of a at 2000° K in the range of n,, t rom 10 1 2 to 10 15 cm- 5 , We have

extrapolated the measured value of a at

T = 2000° K, ti e = 1016cm-3

1he extrapolated value of a, thus determined, is 10-g

cm sec-1,

Recombinations .onipete rather favorably with the two-photon ionization

proce-s. However, the presence of an electric field in the interelectrode

region would tend to keep the ions and electrons separated and make the

r :onlbination process more difficult.

ACKNOWLEDGMENT

The authors would like to acknowiedge the ab:,z assistance of Paul Cassell in

perfort-ning experiments with the converter that he fabricated.

V. CONCLUSIONS

The analysis shows that the plasma density would have to be unreasonably high

for a three photon absorption process to be important in accounting for the

results obtained. Also the absorption of one photon quantum does not corre-

sp, N nd to any energy level difference in cesium. Therefore, the simultaneous

absorption of tNO ruby laser photons appears to be one of the important niecha-

nisms in the ionization process; from Table 1 it is evident that this type of

JPL Technical Memorandum 33-730

absorption will produce cesium atoms excited close to their ionizationlevels.

The excited cesium atoms become ionized as a result of subserfAr , 4% colli-sions by energetic electrons. The cross section fo- inverse hretnsstrahlunginterac' .uns of electrons and ruby photons iu large enough so that energeticelectrons will exist in abundance. It is likely that collisions of the ener-getic electrons with cesium atoms, particularly the excited ones, are aniniportant step in the final ionization mechanism.

8 JPL Technical Memorandum 33-'30

I

Table 1. Energies Involved in Absorptions in Cesium

Proper t y Energy(ev)

Work function of CS 1. 81

1st excitation potential of Cs

6 2 S 62P 1.43

3, 43

3. 57

6 2 S 92P'transi tion in Cs

6 2 S -y 10 2 P

6 2 S - 9 2 D 3. 58

6 2 S - 11 2 P 3. 65

6 2 S -- 16 2 P 3. 81

Ionization potential of Cs, withrespect to ground state

3. 46

L Fnurgy per r)hoton of ruby laser light I 1. 79

:'able 2. Conversion Factors

V

I 6. 250 x 10 1 2 elect ions per microcoulomb

°. 490 x 10 18 photons per joule in ruby laser light

3. 490 x 10 22 photons per joule per second for a 100 - µsec pulse

3. 490 x 10 24 photon flux (no cm -2 sec - 1) for photons crossing

t 1nm 2 area

JPJ. Technical 'Memorandum 33 -730 9

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REFERENCES

1. 1. D. Abella, Physical Review Letters 9,453 (1962)

2. B. A. Tozer, Physical Review, 137, A1665 ( 1965)

3. B. A. Tozer, Op. cit.

4. R. D. Kerr, P. R. Smy, and A. A. Offenburger, Jour. Appl. Phys. 44,3063 (1973)

5. L. Spitzer, Jr. , Physics of Fully Ionized Gases, 2nd Revised Edition, P148.Irterscience Publishers, New York (1962)

6. C. J. Chen, J. M. Wu, F. T. Wu, and 1). T. Shaw, Jour. App!.Phys. 44, 3052 (1973)

JPI. Technical Memorandum 33-730

IJNASA—JPL—Coml.. L A. Calif.